1. Field of the Invention
This invention relates to an image processing method and image processing system for image-processing digital image data acquired by a digital camera.
2. Description of the Related Art
The digital electronic still camera (hereinafter “digital camera”) is internally equipped with a memory, IC card or other such recording medium for storing photographed images as digital image data. The photographed images can be printed by a printer or displayed on a monitor using the recorded digital image data. The images obtained by printing images acquired with a digital camera in this manner should desirably be of a high quality comparable to those printed from negative film.
On the other hand, a common practice has been to convert a negative film image to digital image data, normalize the image data using normalization conditions determined beforehand, and then use the normalized image data to determine the exposure when photographing a subject image on negative film. This technique is adopted because, among the characteristics of negative film, the three-color balance varies with the negative density and this variation causes the gray of the subject to exhibit a different balance in the negative image. The conditions of the three color densities of the gray are therefore determined and used for conversion (normalization) and the converted image data are used to decide the exposure. Further, in the case of using a printmaking process scanner to color-separate a color original image, converting the separated colors to screen dot signals and using the screen dot signals to conduct image printing, the color-separation conditions are corrected to obtain a printed image of appropriate color, density and gradation. Japanese Unexamined Patent Publication No.3(1991)-219777 teaches a method for generating a normalization curve for conducting the correction automatically. This method determines a reference density point for each color component based on the densities of highlight and shadow portions in the original image, generates a gradation conversion curve (normalization curve) to give the density values prescribed signal levels, and corrects the color-separation conditions by using the gradation conversion curve to convert the image data. The distribution of density values of the light portions and dark portions and the tint of the image data can be corrected by this conversion.
Since the digital camera has autowhite balance (AWB) capability, autoexposure (AE) capability and image processing capability, digital image data acquired with the digital camera can be reproduced as a visible image by a printer or other reproduction unit without further processing. However, the AWB and AE capabilities perform differently in different cameras. Moreover, some cameras lack one or both of these features. The reproduction unit is therefore unable to produce printed images of consistent quality.
In light of this, it might be considered helpful to correct the digital image data as taught by JP-A-3-219777 mentioned above. There is, however, no assurance that the digital image data were acquired with the digital camera under proper exposure conditions and, moreover, the shadow portion image information is coarse owing to its high noise content. It is therefore not possible to reproduce high-quality images using digital image data read from an original image or a negative film photographed under proper exposure conditions as a print original and merely corrected by the conventional method.